<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        /* 居中 */
        /* .box{
            width: 200px;
            height: 200px;
            border-radius: 20px;
            background-color: aqua;
            position: absolute;
            top: 50%;
            left: 50%;
            transform: translate(-50%,-50%);
            
        } */
        /* 三角形 */
        .box{
            width:0;
            height:0;
            border:100px solid transparent; border-bottom-color:red;
        }
    </style>
</head>
<body>
    <div class="box"></div>
</body>
</html>